The present invention relates generally to valves of the type used in the petrochemical, refining, and other industries, and more particularly to an improved valve stem which is resistant to chemical and mechanical deterioration, to a method of manufacturing the valve stem, and to an improved packing construction useful in conjunction with the valve stem.
Perhaps the most common valves used in the petrochemical, refining, and other industries are gate valves. Major components of gate valves include a body, a valve stem, a sealing member, and seat rings which are engaged by the sealing member to close the valve. Because the valve stem must extend from inside the valve body to outside the body, a stem packing must be installed to effectively seal between the body and the valve stem, thereby containing the product inside the piping system of which the valve is a part. To achieve the necessary dynamic seal to both contain the product within the piping system and allow the valve stem to actuate up and down, the area of the valve stem which makes contact with the stem packing must be smooth, round and have no taper on the outside surface of the valve stem.
When the valve is new, the foregoing conditions are met and product inside the piping system is prevented from leaking to the outside. As the valve ages, chemical corrosion from the product, galvanic corrosion from the packing, and erosion and corrosion from atmospheric conditions all combine to reduce the smoothness of the valve stem that is necessary to maintain the seal. Also, roughness of the valve stem can occur as the valve stem is pulled through the stem packing during normal cycling due to abrasive materials accumulating between the valve stem and the seal. Any of these factors, and particularly combinations thereof, can result in leak paths being created and leakage to occur. This is unsatisfactory for numerous reasons, the most important of which involves increasing enforcement of the federal Clean Air Act which makes prevention of leakage or xe2x80x9cfugitive emissionsxe2x80x9d essential.
Previous attempts at solving the foregoing problems have included the application of the material sold under the trademark xe2x80x9cHASTALLOYxe2x80x9d(copyright). Such material is partially effective in slowing valve stem deterioration due to chemical attack but is ineffective as to mechanical degradation of the valve stem surface.
While the prior art discloses a variety of valves, no prior disclosure shows a device where leakage is prevented by way of an overlay on the valve stem in the area of the stem packing. The improved valve stem of the present invention departs from the design of prior valve stems, and in doing so, provides a valve which prevents leakage and fugitive emissions as the valve ages.
The present invention provides an improved valve in which the valve stem has an overlay of hardfaced material that reduces the deterioration of the stem packing sealing surface. The hardfaced overlay is constructed of materials which resist corrosion, erosion, and scratching, thus eliminating the cause of most defects in the sealing surface of the valve stem. This in turn allows maintaining the original packing area finish, thereby preventing the creation of leak paths through the stem packing and fugitive emissions from the valve.
In accordance with more specific aspects of the invention, the portion of the valve stem that engages the sealing material has a nominal outside diameter which is first reduced, either during manufacture or modification of the valve stem. Hardfaced material is applied to the reduced diameter portion of the valve stem until the diameter thereof is increased beyond the nominal size. The diameter of the hardfaced material is next reduced to the nominal size, thus providing a valve stem that is smooth, hard, and resistant to chemical and mechanical deterioration.
The invention further comprises an improved valve packing useful in conjunction with the valve stem. The improved valve packing comprises alternating metal pressure rings and graphite seals. The improved packing enhances the performance of the improved valve stem by preventing leak paths through the packing which would otherwise result in fugitive emissions.